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Article

The Impact of COVID-19 Lockdowns on Air Quality—A Global Review

by
Abdullah Addas
* and
Ahmad Maghrabi
Landscape Architecture Department, Faculty of Architecture and Planning, King Abdulaziz University, P.O. Box 80210, Jeddah 21589, Saudi Arabia
*
Author to whom correspondence should be addressed.
Sustainability 2021, 13(18), 10212; https://doi.org/10.3390/su131810212
Submission received: 27 July 2021 / Revised: 26 August 2021 / Accepted: 9 September 2021 / Published: 13 September 2021
(This article belongs to the Collection Impact of COVID-19 on the Environment, Energy and Economics)
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Abstract

:
The outbreak of the COVID-19 pandemic has emerged as a serious public health threat and has had a tremendous impact on all spheres of the environment. The air quality across the world improved because of COVID-19 lockdowns. Since the outbreak of COVID-19, large numbers of studies have been carried out on the impact of lockdowns on air quality around the world, but no studies have been carried out on the systematic review on the impact of lockdowns on air quality. This study aims to systematically assess the bibliographic review on the impact of lockdowns on air quality around the globe. A total of 237 studies were identified after rigorous review, and 144 studies met the criteria for the review. The literature was surveyed from Scopus, Google Scholar, PubMed, Web of Science, and the Google search engine. The results reveal that (i) most of the studies were carried out on Asia (about 65%), followed by Europe (18%), North America (6%), South America (5%), and Africa (3%); (ii) in the case of countries, the highest number of studies was performed on India (29%), followed by China (23%), the U.S. (5%), the UK (4%), and Italy; (iii) more than 60% of the studies included NO2 for study, followed by PM2.5 (about 50%), PM10, SO2, and CO; (iv) most of the studies were published by Science of the Total Environment (29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), and Environmental Pollution (5%); (v) the studies reveal that there were significant improvements in air quality during lockdowns in comparison with previous time periods. Thus, this diversified study conducted on the impact of lockdowns on air quality will surely assist in identifying any gaps, as it outlines the insights of the current scientific research.

1. Introduction

The novel coronavirus (SARS-CoV-2) originated from Wuhan in December 2019, and later spread to many countries across the world [1]. The World Health Organization (WHO) declared this virus a “global pandemic” on 11 March 2020. In many parts of the world, severely affected countries imposed lockdowns to prevent the transmission of COVID-19 by restricting transportation, economic, and industrial activities. Thus, restrictions on human activities and the various productive activities of industries and farms resulted in unforeseen impacts and improved the health of the environment to a great extent. The air quality also significantly improved across the globe because of the restricted emissions from different sources during lockdowns. In many previous research studies, it is well-documented that many countries of the world are facing serious public health problems due to extreme air pollution [2,3,4,5]. More than 60% of the populations living in urban areas are severely exposed to the serious problem of air pollution [1]. Both high-income (56%) and low-income (98%) countries of the world fail to meet the guidelines proposed by WHO [1]. As per the reports published by WHO [1], more than 4.2 billion people have lost their lives because of health risks related to air pollution [1]. Air pollution has become one of the most significant health risks [6,7,8,9] and results in great loss of life. According to an estimation by the Global Burden of Disease Project of the WHO, 1.1 million premature deaths were reported in 2016 primarily because of outdoor particulate matter (PM2.5) pollution [10]. The WHO [1] recently released a report from the Global Ambient Air Quality Database on the concentration of PM2.5 across 100 countries of the world and it was observed that the concentration of PM2.5 was relatively higher across the cities of developing countries, such as India, Bangladesh, Pakistan, the Middle East, Afghanistan, and Mongolia. Developing cities, such as Delhi (India), Dhaka (Bangladesh), Kabul (Afghanistan), Manama (Bahrain), and Beijing (China) are vulnerable to extreme air pollution.
The recent outbreak of COVID-19 has had an immense impact on air quality across the world [11,12,13,14,15]. After the outbreak of COVID-19, a large number of studies were performed on the impact of lockdowns on air quality [16,17,18]. The concentration of major air pollutants, such as PM2.5, PM10, SO2, CO, and NO2, were reduced by about 30% because of lockdowns [19]. Similar studies were conducted in Spain [20], Italy [21], Brazil [22], (Morocco [23], India [24], the U.S. [25], and Bangladesh [26]. All the studies concluded that there was a significant reduction in air pollutants, and a significant improvement in air quality during lockdown due to COVID-19.
According to Muhammad et al. [27], there were substantial decreases in fuel demand around the world during lockdown periods due to the cessation of transportation and industrial activities. The reduced demand for fuel resulted in the drastic reduction of carbon emissions and air pollutant concentrations [15,16,17,28,29]. To the best of our knowledge, there have been no review studies performed on the impact of lockdowns on air quality across the globe until now. Considering this research gap, this study aims to assess a review on the impact of lockdown on air quality across the world. This study includes more than 40 countries across the world from six continents, and more than 140 research studies. This is the first attempt at dealing with the assessment of a systematic review on the impact of COVID-19 lockdowns on air quality on a global scale. The findings of this study could help planners and policymakers understand, as well as implement, effective strategies for the reduction of air pollution levels at the city, regional, and country scales.

2. Materials and Methods

At the beginning of the study, a total of 237 research studies were searched using keywords. From the 237 research studies, 144 research papers were finally selected on the basis of two criteria. In the initial stages, the studies were shortlisted through the screening of the abstracts and titles of the papers. At the second stage of the literature screening, the articles were selected on the basis of the scales of the studies (the city, regional, and country scales). Thus, 144 articles were finally included in this literature screening. Before the final selection of studies for this review assessment, some criteria were set. First, the studies performed on the impact of lockdowns on the air quality at the city, regional, and country scales were included in this review process. Secondly, the literature screening was carried out from March 2020 to April 2021, and no studies published after April 2021 were considered. Thirdly, the studies performed on multiple countries, or cities from multiple countries, were not considered for the review process. The details of the literature screening procedures are presented in Figure 1. Previous literature was considered on the basis of the objectives of the study (Table 1 and Table 2), i.e., studies performed on the impact of lockdowns on the air quality at the city, regional and country scales, respectively. The entire process of the literature screening is shown through a PRISMA flow diagram [30] (Figure 2).

Keywords for Search of Academic Databases

The main objective of this study was to examine the impact of COVID-19 lockdowns on air quality. Thus, keywords, such as “air pollution”, “air quality”, “lockdown”, “COVID-19”, and “pandemic” were used. The main databases searched were Scopus, Web of Science, Science Direct, Google Scholar, PubMed, and the Google search engine. The majority of the articles were searched from Google Scholar, followed by Scopus, Web of Science, and the Google search Engine, from March 2020 to April 2021 (Figure 1).

3. Results

3.1. Geographical Distribution and COVID-19 Studies

In the present analysis, about 70% of the total papers were published in 2020 (particularly from March to December) and the rest of the papers were published (30%) in 2021 (considered up to the month of April). This study encompasses five continents: Asia, Europe, Africa, North America, and South America. About 65% of the total studies were surveyed from Asia, followed by Europe (18%), Africa (7%), North America (6%), South America (6%), and Australia (1%). In the case of countries, the highest number of studies were surveyed from India (29%), followed by China (23%), the U.S. (6%), the UK (5%), Italy (3.8%), and Bangladesh (2.78%) (Figure 3).

3.2. Impact of COVID-19 on Air Quality over Asian Countries

From the overall studies surveyed, it was observed that the greatest number of studies on the impact of the COVID-19 pandemic on air quality was performed on Asian countries (65%). As per our literature survey, it is documented that about 45% of the research studies on the impact of COVID-19 on air quality was identified from India, followed by China (36%), Bangladesh (4%), Pakistan (2.6), Korea (2.6%), and Thailand (1.7%). In India, most of the studies were performed on polluted cities and large megacities, such as Delhi, Mumbai, Kolkata, Chennai, Bengaluru, Varanasi, Patna, Gaya, Hyderabad, and Pune. About 60% of the studies were performed over the megacity, Delhi (the capital city of India). In China, most of the studies were performed on large cities, such as Wuhan, Beijing, the megacity Hangzhou, Anqing, Hefei, and the city of Suzhou (Table 3). The studies focusing on the concentrations of PM2.5 are shown in Figure 4.

3.3. Impact of COVID-19 on Air Quality over European Countries

After Asia, the second highest number of studies was performed on European countries. As per the literature survey, 33% of the total studies were collected from European countries. In this review, studies were collected from 13 countries in Europe with the highest number of studies on the UK (23%), followed by Italy (20%), Spain (20%), Turkey (8.8%), and France (5.5%). The cities in Europe on which studies were performed were Barcelona, Madrid, Naples, the city of Novi Sad, Munich, Tirana, Southampton, and Milan. As per the literature survey, most of the studies were performed on the concentrations of NO2 (more than 50% of the study). As per the results of Anderson and Dirks [114] (2020), Lee et al. [115], and Jephcote et al. [116], the concentration of NO2 decreased by about 92%, 42%, and 38%, respectively, in the UK Similarly, in Italy, the concentration of NO2 declined by about 49–62%, and SO2 decreased by about 70% [117] during lockdown periods. Thus, from the previous studies, it is clear that there was a substantial decline in air pollutants, and air quality significantly improved during lockdown periods. The details of the impact of lockdown on air quality are presented in Table 4. Most of the studies focusing on the concentration of PM10 are presented in Figure 5.

3.4. Impact of COVID-19 on Air Quality over North American Countries

In North America, the greatest number of studies on air quality were performed on the United States and Canada. In this literature review, about 90% of the total studies were surveyed from the U.S., followed by Canada (10%). Most of the studies in the U.S. were performed at the national level and the city scale (such as California and New Jersey). In the U.S., there were substantial reductions in air pollutants due to lockdown. For example, according to Goldberg et al. [146], NO2 declined by about 9% to 42%, with the highest decline in San Jose and Los Angeles, and the lowest decrease (<12%) in Miami, Minneapolis, and Dallas. As per the study of Jiang et al. (2020), PM2.5 concentration was reduced by more than 68% after lockdown. Moreover, other studies in the U.S. also revealed that there was a significant decrease in the air pollutant concentrations during lockdown periods that resulted in a substantial improvement in air quality (Table 5). The studies focusing on the assessment of CO are shown in Figure 6.

3.5. Impact of COVID-19 on Air Quality over South American Countries

In South America, the greatest number of studies were performed on Brazil, Ecuador, and Mexico. In this review, about 40% of the total studies were surveyed from Brazil and Ecuador, followed by Mexico (20%). As per the results of the study, it was documented that there was a substantial improvement in air quality during lockdown periods. According to Hernández-Paniagua et al. [154], the concentration of NO2 was reduced by between 10% and 35% in Mexico during lockdown periods. Zalakeviciute et al. [155] performed a study in Quito (Ecuador), and the findings of the study show that air quality improved by about 26% to 68%. According to Nakada and Urban [156], NO, NO2, and CO decreased by about 70%, 50%, and 60%, respectively, in Sau Paulo (Brazil). The details of the findings on the impact of lockdown on air quality are presented in Table 6. The studies focusing on the assessment of NO2 are shown in Figure 7.

3.6. Impact of COVID-19 on Air Quality over African Countries

In this review, four countries were surveyed from Africa, with the highest percentage of studies on Morocco (42%), followed by Egypt (28%), Uganda (14.3%), and Nigeria (14.3%) (Table 7). In African countries, there were also significant improvements in air quality during lockdown periods. For example, as per the study by Otmani et al. [162], the concentration of PM2.5 and PM10 decreased by about 75% and 96%, respectively, during lockdown periods. Similarly, CO and NO2 decreased by about 46% and 45%, respectively. The studies focusing on the assessment of SO2 are shown in Figure 8.

3.7. Number of Publications and Journal Distributions

As per literature screening from different sources, it was observed that there were more than 300 studies performed on the impact of lockdown on air quality across the world (as per our observations from April 2020 to March 2021). Among all the countries, the greatest number of studies were performed on Asian countries, followed by European countries. From the literature screening, it was well-recognized that the greatest number of research studies were published by Science of The Total Environment (about 29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), Environmental Pollution (6%), and Environmental Research (4%). From the top ten journals, about 80% of the total studies were included (Figure 9 and Figure 10).

4. Discussion

The present study mainly focuses on a review on the impact of COVID-19 lockdowns on air quality around the globe. From the results, it was found that most of the studies on the impact of lockdowns on air quality were performed in Asian countries (about 65%), followed by European countries (18%), and North and South American countries (10%). As per the literature screening, it was reported that the greatest number of studies on the impact of lockdown on air quality were reported from India (29%), followed by China (23%), the UK (4.44%), and Italy (3.89%). From the previous research studies, it was recognized that there was a strong association between air pollutants and respiratory disease [168,169,170]. Thus, the areas with high exposure to air pollution are vulnerable to mortalities due to respiratory diseases. With COVID-19 being a respiratory disease, it is obvious that COVID-19 deaths are strongly influenced by air pollutants. Recent studies also show that the concentrations of air pollutants are significant risk factors in COVID-19 deaths [171,172,173,174]. In many studies across the cities of the world, it is reported that concentrations of air pollutants are strongly associated with COVID-19 cases, as well as deaths. The severely affected countries of the world imposed several measures to fight COVID-19 and reduce the transmission of the virus around the world [47]. China was the first country to implement a complete shutdown in commercial fields, restrictions on domestic and international travel, and strict COVID-19 protocols were imposed across affected cities [175]. After that, similar restrictions were implemented by several other countries, such as India, Italy, and France. These restrictions were placed on public transportation, social gatherings, schools and colleges, and emissions from industries as well (Das et al., 2020). Economic activities are the prime factors that contribute to environmental pollution because of the combustion of fuel and the release of air pollutants into the atmosphere [175]. In most of the densely populated cities, particularly the ones in developing countries, air pollution levels are higher than the tolerance limits, and that results in risks to human health. In urban areas, the concentration of primary air pollutants, such as CO, NO2, SO2, O3 are significantly higher. The concentrations of particulate matters (PM2.5 and PM10) were significantly decreased during lockdown periods due to the restriction on emissions from various sources [18]. A substantial improvement in air quality was reported for most polluted cities around the world, such as Delhi (the capital city of India, India), Dhaka (the capital city of Bangladesh), and Beijing (China). Thus, COVID-19 lockdowns have had positive impacts on the environment. Therefore, from the overall results, it can be stated that lockdown, particularly short-term lockdown, can, to some extent, be considered an alternative measure to reduce air pollution level. The outbreak of COVID-19 compelled the affected countries to impose lockdowns to curb COVID-19 transmission, and many countries have remained under partial lockdown since last year. Thus, the environment has had a long time to restore its capacity because of the cessation of many economic activities. However, in reality, it is not possible to completely cease economic activity because it is a matter of human livelihoods. So, planners and policymakers must implement and follow sustainable strategies to reduce air pollution levels. The lockdowns during COVID-19 not only induced the improvement of air quality, but it has also had other positive effects on the environment. Measures regarding social distancing kept people away from resorts and sea beaches, and effluent discharge into the water stopped because of industrial shutdowns. Water quality also improved in many countries, such as in Spain, Equador, and Mexico. For instance, as per the findings of Paital et al. (2020), and Saadat et al. (2020), the water quality of Venice’s canals, and the Yamuna River in Delhi, improved significantly in comparison with past years. Apart from environmental perfectives, COVID-19 lockdowns also had a substantial impact on the global economy. As per a World Trade Organization (WTO, 2020) report, economic activity decreased in world trade between 13% and 32%. Thus, it has been well-documented in previous literature that COVID-19 lockdowns result in substantial positive, as well as negative, impacts on the environment.
The present review study on the impact of COVID-19 lockdowns on air quality across the globe can provide a unique work for understanding, as well as implementing, effective strategies at the city, regional, and country scales. For example, among all the regions, Asian countries, such as India, China, and Bangladesh, are more severely affected by extreme air pollution levels. At the country scale, most of the studies were performed in India, followed by China, and the UK Thus, at the global scale, effective strategies can be implemented for the Asian region to fight air pollution levels, and at the country scale (such as with India), cities can be identified through this literature and strategies for them can thus be implemented. Therefore, this study may be very helpful to planners and policymakers for understanding the global scenario and the improvement of air quality due to COVID-19 lockdowns. In this study, few limitations can be identified. Firstly, in this study, the literature was reviewed from March 2020 to April 2021. No studies were taken into consideration that were published after April 2021 in this review assessment. Thus, further research in the future can be performed on literature published after April 2021 for a better understanding of the impact of lockdown on air quality. Secondly, no models or laws were applied in this study for literature screening. Thus, for future researchers, we suggest implementing the Bradford law for literature screening. In spite of these limitations of the study, this is a unique piece of research into the global scenario with respect to the impact of COVID-19 lockdowns on air quality.

5. Conclusions

In this study, an attempt has been made to examine and review the impact of COVID-19 lockdowns on air quality on a global scale. Initially, 237 studies related to the impact of COVID-19 lockdowns on air quality were screened, and 144 studies were finally taken into account for this literature review. The literature was extracted from Scopus, Google Scholar, PubMed, Web of Science, and the Google search engine. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) was also used for the literature screening. As per the results of this study, it is well-documented that most of the studies were performed over the Asian region (65%), followed by Europe (18%), North America (6%), South America (5%), and Africa (3%). At the country scale, the greatest number of studies were conducted in India (29%), followed by China (23%), the U.S. (5%), the UK (4%), and Italy. As per our investigation, it was observed that the greatest number of studies on the impact of the COVID-19 pandemic on air quality was published in Science of the Total Environment (29%), followed by Aerosol and Air Quality Research (23%), Air Quality, Atmosphere & Health (9%), and Environmental Pollution (5%). From the core findings of the literature, it is apparent that there was substantial improvement in air quality due to lockdowns across the world. For example, Naqvi et al. [19] performed a study on India and reported that the air quality index was reduced by about 40% during one month of lockdown. According to Filonchyk et al. [76], and Diamond and Wood [82], the concentrations of CO and NO2 were reduced by about 20% and 30%, respectively, and by 50% in China. In the UK, NO2 was reduced by about 42% during lockdown periods [107]. Thus, in all of the countries, it has been well-recognized that there was a substantial improvement in air quality during the lockdowns. The implementation of lockdowns around the world resulted in the improvement of the air quality and provided us with an opportunity to realize the impact of the anthropogenic pressures on the environment. Thus, the findings of the review will surely assist planners and policymakers to understand that the implementation of lockdowns may be an effective measure to restore the environment, and to build quality ecosystems in urban environments. All the affected countries of the world imposed effective measures to try to slow down the transmission of COVID-19. These measures included the closures of industrial activities, strict restrictions on transportation, and the cessation of other productive activities that resulted in the improvement of air quality.

Author Contributions

Methodology, A.A. and A.M.; software, A.A. and A.M.; formal analysis, A.A. and A.M.; resources, A.A. and A.M.; data curation, A.A. and A.M.; writing—original draft preparation, A.A.; writing—review and editing, A.A. and A.M.; visualization, A.A. and A.M.; supervision, A.A.; funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy reasons.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Details of the methodology used in this literature review.
Figure 1. Details of the methodology used in this literature review.
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Figure 2. PRISMA flow diagram showing procedure used for systematic review.
Figure 2. PRISMA flow diagram showing procedure used for systematic review.
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Figure 3. Global scenario of air quality studies (from March 2020 to April 2021).
Figure 3. Global scenario of air quality studies (from March 2020 to April 2021).
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Figure 4. Studies focusing on PM2.5 concentrations.
Figure 4. Studies focusing on PM2.5 concentrations.
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Figure 5. Studies focusing on PM10 concentrations.
Figure 5. Studies focusing on PM10 concentrations.
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Figure 6. Studies focusing on CO concentrations.
Figure 6. Studies focusing on CO concentrations.
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Figure 7. Studies focusing on NO2 concentrations.
Figure 7. Studies focusing on NO2 concentrations.
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Figure 8. Studies focusing on SO2 concentration.
Figure 8. Studies focusing on SO2 concentration.
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Figure 9. Contribution of major journals as per the literature screening (%).
Figure 9. Contribution of major journals as per the literature screening (%).
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Figure 10. Top 10 journals as per the literature screening (%).
Figure 10. Top 10 journals as per the literature screening (%).
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Table
Table 1. Distribution of literature across major continents (until April 2021).
Table 1. Distribution of literature across major continents (until April 2021).
ContinentsCountryNumber of Studies
Europe (33)UK (8), Turkey (3), France (2), Spain (6), Italy (7), Germany (1), Poland (1), Netherland (1), Portugal (1), Russia (1), Macedonia (1), Albania (1), 33
North and South America (20)USA (9), Canada (1), Ecuador (4), Brazil (4), Mexico (2), 20
Asia (117)India (53), China (42), Thailand (2), Bangladesh (5), Malaysia (2), Singapore (1), Iran (1), Israel (1), Japan (1), Pakistan (3), Vietnam (1), Korea (3), Kazakhstan (1), Saudi Arabia (1) 117
Oceania (2)Australia (2)2
Africa (7)Nigeria (1), Morocco (3), Egypt (2), Uganda (1)7
Table
Table 2. Country-wide distribution of the literature across the world.
Table 2. Country-wide distribution of the literature across the world.
ContinentCountryNumber of Studies% of Studies
AsiaIndia5329.44
China4223.33
Bangladesh52.78
Thailand21.11
Pakistan31.67
Malaysia21.11
Korea31.67
Israel10.56
Iran10.56
Vietnam10.56
Kazakhstan10.56
Saudi Arabia10.56
Teheran10.56
Singapore 10.56
EuropeUK84.44
Spain63.33
Italy73.89
Turkey31.67
Russia10.56
Germany10.56
Macedonia10.56
Albania10.56
Portugal10.56
Netherlands 10.56
Poland 10.56
Serbia10.56
France21.11
North AmericaUS95.00
Canada10.56
AfricaMorocco31.67
Egypt21.11
Kampala10.56
Nigeria10.56
South AmericaBrazil42.22
Ecuador42.22
Mexico21.11
OceaniaAustralia10.56
Table
Table 3. Literature on the impact of COVID-19 on air quality in Asian countries.
Table 3. Literature on the impact of COVID-19 on air quality in Asian countries.
CountryStudy AreaPublication YearMajor Findings
IndiaCity scale2020A substantial decrease in PM2.5 and the air quality index (AQI) was reported for Delhi, Mumbai, Hyderabad, Kolkata, and Chennai. (ii) PM2.5 concentrations were reduced by 34.52% and 27.57% in Kolkata and Delhi, respectively, in comparison to 2019 [18].
Country2020There was a remarkable decline in the ambient air quality index (AQI) (17.75% and 20.70%, respectively) during post-lockdown periods as compared to pre-lockdown periods (ii) poor air quality had a positive correlation with COVID-19 mortalities (r = 0.435 for AQI) [31].
State2020There was a substantial reduction in air pollutants during different phases of lockdowns (ii) PM 2. 5 and PM10 decreased by about 17.76% and 20.66%, respectively, during consecutive periods of lockdowns [32].
City scale2021PM 2.5 was reduced by about 40 to 45% during lockdown periods in comparison to the previous two years [33].
City scale2020Particulate matter concentration decreased by about 40% during lockdown in comparison to previous years [34].
City scale2020The lockdown measures reflected a significant reduction in air pollutants; the most significant fall was estimated for NO2 (29.3–74.4%), while the least reduction was noticed for SO2 [35].
City scale2020The average value of AQI at Punjab Bagh was noticed as 212 before the lockdown, which dropped down to 74 during the lockdown, indicating a significant improvement in air quality [23].
City scale2020The results indicate the lowering of PM 2.5, PM 10, and NO2 concentrations in the city by 93%, 83%, and 70%, respectively, from 25 February 2020 to 21 April 2020 [17].
City scale2020The concentration of NO2 and PM2.5 significantly decreased due to lockdowns across cities [36].
City scale2020These two cities observed a substantial decrease in nitrogen dioxide (40–50%) compared to the same period last year [37].
City scale2020Major negative effects on the social and surrounding environment have been reported due to COVID-19, however positive effects have also been observed with respect to air quality. The results have been taken from the National Aeronautics and Space Administration (NASA), and indicate a significant reduction (50%) in the air quality of the Indian region [23].
City Scale2020A considerable reduction (∼30–70%) in NO2 was found, except for a few sites in the central region. A similar pattern was observed for CO having a ∼20–40% reduction. The reduction observed for PM2.5, PM10, NO2, and the enhancement in O3 was proportional to the population density [38].
City scale2021PM2.5 has declined by 14%, by about 30% for NO2 in million-plus cities, and a 2.06% CO, SO2 within the range of 5 to 60%, whereas the concentration of O3 has increased by 1 to 3% in the majority of cities compared with pre-lockdown. On the other hand, CPCB/SPCB data showed a more than 40% decrease in PM2.5 and a 47% decrease in PM10 in north Indian cities, more than a 35% decrease in NO2 in metropolitan cities, more than an 85% decrease in SO2 in Chennai and Nagpur, and a more than 17% increase in O3 in five cities during 43 days of pandemic lockdown [39].
City scale2020The lockdown effect due to COVID-19 in the city: the complete closure of industries, transports, markets, shopping malls, recreation units, construction works, etc., which are the main sources of CO2 emissions [40].
City scale2021Highest levels of PM10 and PM2.5 were observed near sunrise, with little change in the time of maximum levels between 2019 and 2020 [41].
City scale2020A reduction of almost 60% in the particulate matter pollution, and up to 40% in the NOx pollution, were observed, while the ozone levels were reduced by 30–40%, as compared to the same period during the previous two years [42].
City scale2021The air quality has improved across the country and the average temperature and maximum temperature were connected to the outbreak of the COVID-19 pandemic [43].
City scale2020Before 30 days of lockdown, PM2.5 was 65.77 µg/m3 and that reached 42.72 µg/m3 during lockdown periods [44].
City scale2021 (a)During lockdown, maximum decrease was reported for NO2 (40%), followed by PM2.5 (32%), PM10 (24%), and SO2 (18%) [45].
City scale2021 (b)During entire periods of lockdown, the average concentration of PM2.5 declined by 50% [46].
City scale2020Suspended particulate matter (SPM) was reduced by about 36%. The concentration of NO2 was also reduced during lockdown periods [46].
City2020The concentration of PM2.5, PM10, and NO2 declined by about 50%, with a significant increase in O3 in Delhi (p < 0.05) [35].
Country2021Over the urban agglomerations (UAs), and rural regions, the concentrations of NO2 were reduced by about 20–40% and 15–25%, respectively [47].
Regional2020Mumbai recorded the highest decrease of NO2 (34%) with a seasonal decrease of SO2 in western and southern India [48].
City2021During lockdown periods, the concentration of PM 2.5 and PM10 declined by about 43% and 59%, respectively, in Delhi, and by 50% and 49%, respectively, in Kolkata [49].
City2020During the initial periods of lockdown, the concentration of PM 2.5 declined by about 40 to 70% (from 25 March to 31 March 2020) [50].
City2020From 11 May to 9 June 2020, the concentrations of PM2.5, PM10, and NO2 were reduced by about 74%, 46%, and 63%, respectively [51].
City2020There was a substantial decrease in PM 2.5, PM10, and NO2 during lockdown, with the highest decline in Ahmedabad (68%), Delhi (71%), Bangalore (87%), and Nagpur (63%), for PM2.5, PM10, NO2 and CO, respectively [16].
City2020NO2 was reduced by about 46% and the air quality index (AQI) improved by about 27% [52].
City2020Air quality index (AQI) was reduced by 44, 33, 29, 15, and 32% in north, south, east, central and western India. The highest decrease was reported for PM2.5 (43%), followed PM10 (33%), NO2 (18%), and CO (10%) [53].
City2020Air pollutants (PM2.5, PM10, NO2, and CO) were reduced by about 50% across the megacities of India [54].
City2020The concentration of PM2.5 was reduced by about 19 to 43% in Chennai, 41 to 53% in Delhi, 26 to 54% in Hyderabad, 24 to 36% in Kolkata, and 10 to 39% in Mumbai [55].
City2020The concentrations of PM2.5, PM10, NO2 and SO2 were 49, 55, 60 and 19%, respectively, in Delhi, and 44, 37, 78, and 39%, respectively, in Mumbai [56].
City2020PM10 was reduced by more than 46% across five cities [57].
City2020Over the urban agglomerations (UAs) and rural regions, the concentrations of NO2 were reduced by about 20–40%, and 15–25%, respectively [58].
City2021The concentrations of PM10, PM2.5, NOx, SO2, and CO were reduced by about 58, 47, 83, 11, and 30%, respectively [59].
City2020The concentration of PM2.5 decreased from 72.9 μg m−3 (2019) to 45.9 μg m−3 (2020) during lockdown periods [60].
City scale2020The concentrations of PM2.5, PM10, SO2, CO2, and NO2 decreased due to lockdown [17].
Country and City scale2020Air quality improved by about 25% during lockdown periods [61].
City scale2020The over-standard multiples method and a grey relational analysis to study the individual and overall change trends of pollutants in Wuhan during the same period in the past seven years. The results show that the concentrations of SO2 and O3 increased because of the pandemic, but still met the standard [62].
City Scale2020Urban aerosols decreased from 27.1% for pre-C19Q aerosols to only 17.5% during C19Q. WRF-Chem reported a ~0.2 °C warming across east-central China that represented a minor, though statistically significant, contribution to C19Q temperature anomalies. The largest area of warming is concentrated south of Chengdu and Wuhan, where temperatures increased between +0.2–0.3 °C [63].
City scale2021The increment in secondary organic and inorganic aerosols under stationary weather reached up to 36.4% and 10.2%, respectively, which was further intensified by regional transport. PRD was quite the opposite. The emission reductions benefited PRD air quality, while regional transport corresponded to an increase of 17.3% and 9.3% in secondary organic and inorganic aerosols, respectively. In different regions, the maximum daily 8 h average ozone (O3) soared by 20.6–76.8% in YRD but decreased by 15.5–28.1% in PRD. In YRD, nitrogen oxide (NOx) reductions enhanced O3 accumulation and, hence, increased secondary aerosol formation [64].
City scale2020It was found that the COVID-19 pandemic caused PM2.5 and AQI to decrease by about 7 μg/m3 and 5-points, respectively [65].
City scale2021The precipitous decrease of AQI and PCDI in Q1 2020, and the peaks of the AQI during the epidemic period were closely related to people’s activities. AQI, PM2.5, and NO2 were significantly positively correlated with PCDI [66].
City scale2020The average concentrations of PM2.5, PM10, SO2, CO, and NO2 were 89.4 µg m–3, 106 µg m–3, 2.31 ppb, 0.72 ppm, and 12.3 ppb, respectively, and were 17.9%, 30.8%, 83.8%, 19.8%, and 62.1%, lower than those in February from 2017–2019. However, the average O3 concentration was 31.8 ppb in February 2020 [67].
City scale2021PM2.5, PM10, SO2, and NO2 during a 2-week portion of the lockdown period (from 24 January–6 February) were reduced by −19.2%, −44.7%, −21.5%, and −33.6%, respectively, compared to the same period in 2019. Even with the decrease in PM2.5 and PM10 concentrations, they were still more than four times higher than the World Health Organization standards (10μg/m3 and 20 μg/m3, respectively) [68].
City scale2020Average concentrations of PM2.5 and PM10 across China were 10.5% and 21.4% lower, respectively, during the lockdown period. The largest reductions were in Hubei province, where NO2 concentrations were 50.5% lower than expected during the lockdown [69].
City scale2020PM2.5 and PM10 were reduced by about 10%, 12% [70].
City scale2020The AQIs in these cities were brought down by 6.34 points (PM2.5 was down by 7.05 µg m−3) relative to the previous year. The lockdown effects were greater in colder, richer, and more industrialized cities [61].
City scale2020In January (2020), average concentration of PM2.5 and PM10 was 23.8% and 33.9% (over Anqing, Hefei and Suzhou) which was lower in comparison to previous year (2017–2019) [15].
City scale2020The pandemic promoted a decrease in PM2.5, PM10, and NO2 concentrations, but it had just reached the standard or even exceeded the standard [71].
ChinaCity scale2020The concentrations of SO2 and O3 increased but still met the standard. However, the pandemic promoted a decrease in PM2.5, PM10, and NO2 concentrations, but it had just reached the standard or even exceeded the standard [62].
Country and City scale2020 O3 responses to NO2 declines can be affected by the primary dependence on its precursors [72].
City scale2021The air quality index (AQI) during the lockdown period decreased by 7.4%, and by 23.48%, compared to pre-lockdown levels and the identical lunar period during the past 3 years, respectively, which exhibited optimal air quality due to reduced emissions [73].
City scale2020A causal relationship between P and R across 31 provincial capital cities in China was established via matching. A higher P resulted in a higher R in China. A 10 µg/m3 increase in P produced a 0.9% increase in R (p < 0.05). An interaction analysis between P and absolute humidity (AH) showed a statistically significant positive relationship between P × AH and R (p < 0.01). When AH was ≤8.6 g/m3, higher P and AH produced a higher R (p < 0.01) [74].
City scale2021The number of days with NO2, PM10, and PM2.5 as the primary pollutants decreased by approximately 10, 9, and 15%, respectively. We compared the wind direction, wind speed, temperature, and relative humidity from January-April 2020, 2019, 2018, and 2017, and found no obvious correlation between meteorological factors and improved air quality during the 2020 lockdown [75].
Country2020The concentrations of CO and NO2 were reduced by about 20% and 30%, respectively [76].
City2021During lockdown periods, PM2.5 decreased by about 30% and NO2 by 50%, respectively [77].
City2020The concentration of PM2.5, PM10, NO2, and SO2 decreased by about 6, 14, 25, and 7%, respectively [78].
City2021The PM2.5 and SO2 were reduced from 37 to 26 ug/m3 and from 6 to 4 ug/m3, respectively, during restricted lockdown periods [79].
City2020The concentration of PM2.5 was higher during New Year holidays in 2020 (73%) than New Year holidays in 2019 (59%) [80].
Country2020In comparison to last year (2019), the concentrations of CO, NO2, SO2, PM2.5, and PM10 were reduced by about 12, 16, 12, 15, and 14%, respectively [81].
Country2021Lockdown resulted in about a 50% reduction in NO2 [82].
Country2021The NO2 was reduced by about 53, 50, and 30% in Wuhan, Hubei province, and China, respectively. The concentration of PM2.5 declined by about 35, 29, and 19%, respectively, in comparison to last year [83].
Country2020NO2 declined by about 24% during the Chinese New Year (CNY) holiday [84].
Country2020The concentration of NO2 was reduced by about 20 to 50% for cities, 15 to 40% for maritime transport, and 40% for power plants [85].
Regional2020There were reductions of PM2.5 concentration from 22.9% to 43% during lockdown periods, as compared to previous year [86].
City2020A substantial reduction of PM2.5, PM10, CO, and SO2 were reported during lockdown periods [87].
Country2020Air pollution was reduced by up to 90% during city lockdown [88].
Regional2020The concentrations of PM2.5, PM10, and CO decreased by about 40%, 45%, and 24%, respectively, during lockdown periods [89].
Regional2020Lockdown resulted in a substantial reduction in PM2.5 (27–46%), NO2 (29–47%), and SO2 (16–26%) [90].
Regional2020Carbonaceous particles decreased by about 20% during lockdown periods [91].
City2020During lockdown periods, the concentration of PM2.5 and NO2 decreased by about 36% and 53%, respectively, and O3 increased by about 116% [92].
Country2020During lockdown periods, the concentration of PM2.5 decreased by up to 23 ug/m3 [93].
ThailandCity scale2020Air quality improved by about 50% to 70% during lockdown periods due to restricted emissions from transportation [94].
City scale2020 The environmental benefits documented in major urban agglomerations during the lockdown may extend to medium-sized urban areas as well [95].
BangladeshCity scale2021Due to lockdown measures, significant differences between PM2.5, SO2, NO2, CO, and O3 in 2019 and 2020 were observed in Dhaka city. We used lag-0, lag-7, lag-14, and lag-21 days on daily COVID-19 cases to look at the lag effect of different air pollutants on meteorology [25].
City scale2021The concentration of NO2, PM2.5, and SO2 decreased by about 20%, 26%, and 17.5%, respectively, because of lockdown [38].
City scale2021The concentration of PM2.5 and PM10 decreased by 40% and 32% during lockdown periods in comparison to previous dry seasons [96].
Country scale2020The concentration of NO2 and SO2 decreased by about 40% and 43%, respectively [97].
City scale2020Air quality during lockdown was found to be 5.30% lower than 2019 [98].
MalayasiaCountry and City scale2020PM.5 and PM10 decreased by about 25% during lockdown [99].
City scale2020Differences between PM10, PM2.5, SO2, NO2, CO, O3, and solar radiation in 2019 and 2020 since the movement control order (MCO) was implemented on 18 March 2020 [100].
SingaporeCountry and City scale2020The concentrations of the following pollutants PM10, PM2.5, NO2, CO, and SO2 decreased by 23, 29, 54, 6, and 52%, respectively, while that of O3 increased by 18%. The Pollutant Standards Index decreased by 19% [101].
KoreaCity scale2020In March 2020, PM2.5 showed remarkable reductions of 36% and 30% in Seoul and Daegu, respectively, when compared with the same period from 2017–2019 [102].
City scale2020The PM2.5 concentration decreased by about 10.4%, where the average concentration of PM2.5 was 23.7% the last 5 years [103].
Country2021The concentration of PM2.5, PM10, and NO2 declined by about 45, 35, and 20%, respectively, because of lockdown [104].
IsraelCity scale2020In its earlier closest period, the pollution from transport, based on nitrogen oxides, had reduced by 40% on average, whereas the pollution from industry, based on Grand-level ozone had increased by 34% on average [105].
IranCity scale2020PM2.5 increased by 0.5–103, 25, and 2–50%. In terms of the national air quality, SO2 and NO2 levels decreased, while AOD 26 increased during the lockdown [106].
PakisthanCountry2021There were no significant improvements of air quality in Lahore and Karachi during lockdown periods, as compared to 2019 [107].
City scale2021With the reduction in human activity (known to be the biggest source of air pollution) during the COVID-19 pandemic, changes in air pollution values were observed. The year 2020, compared with 2018 and 2019, in order to observe this change and to compare it with other years: 1 January–15 March, considered the pre-pandemic process; 16 March–31 May, considered the pandemic process; 1 June–30 June, considered the normalization process [108].
City2021During lockdown periods, PM10, PM2.5, NO2, and CO were reduced 32–43%, 19–47%, 29–44% and 40–58%, respectively [109].
VietnamCity scale2020The concentrations of NO2, PM2.5, and SO2 were reduced by about 75%, 55%, and 67%, respectively [110].
KazakhstanCity scale2020PM2.5 declined by 21%, and CO and NO2 decreased by about 49% and 35%, respectively, during lockdown [111].
Saudi ArabiaRegional2021The eastern province of Saudi Arabia reported a reduction in PM10, CO, and SO2 by 21–70%, 5.8–55%, and 8.7–30%, respectively [112].
TeheranCountry2020There were increases in PM2.5 and PM10 (by 20.5% and 15.7%) during the first month of the COVID-19 outbreak [113].
Table
Table 4. Literature on the impact of COVID-19 on air quality in European countries.
Table 4. Literature on the impact of COVID-19 on air quality in European countries.
CountryScale of Study Publication YearMajor Findings
UKEngland2021PM2.5 was a major contributor to COVID-19 cases in England, as an increase of 1 m3 in the long-term average of PM2.5 was associated with a 12% increase in COVID-19 cases [118].
Southampton2020NO2 decreased by about 92% during lockdown, as compared with the previous two years [114].
Country2020NO2 was reduced by about 42% during lockdown periods [115].
Country2021The concentration of NO2 and PM2.5 concentrations decreased by 38.3% and 16.5%, respectively [116].
Country2021The concentration of NO, NO2, and NOx decreased 32% to 50% at roadsides during lockdown [119].
Country2021NO2 concentrations across measurement sites declined by about ~14–38% [120].
Country2021The concentration of NO2 decreased by about 50%, and O3 increased by about 10% [121].
Country2021The concentration of Ox emissions declined nationwide by ~20% during the lockdown [122].
SpainCity 2020The 4-week lockdown had a significant impact on reducing the atmospheric levels of NO2 in all cities, except for the small city of Santander, as well as the levels of CO, SO2, and PM10 in some cities, but resulted in an increase of the O3 level [123].
Country2020Changes in the concentration of the pollutant nitrogen dioxide (NO2) during the lockdown period were examined, as well as how these changes relate to the Spanish population [124].
City 2021In 2020, NOx, NO2, and NO concentrations decreased by 48.5–49.8–46.2%, 62.1–67.4–45.7%, 37.4–35.7–35.3%, 60.7–67.7–47.1%, 65.5–65.8–63.5%, 60.0–64.5–41.3%, and 60.4–61.6–52.5%, respectively [125].
Country2021Decreases in PM10 levels were greater than in PM2.5 because of reduced emissions from road dust, vehicle wear, and construction/demolition activities. The averaged O3 daily maximum of 8-h (8hDM) experienced a generalized decrease in the rural receptor sites in the relaxation (June-July) with −20% reduced mobility [20].
Country2020NO2 was reduced by about 50% during lockdown periods [126].
City 2020The concentration of NO2 in Barcelona and Madrid decreased by about 50% and 62%, respectively, during lockdown periods [127].
ItalyCity 2021NO2 decreased by about 50%, 34% and 20% from urban traffic, urban backgrounds, and rural backgrounds, respectively [128].
Regional2020Potentially, it is the spatially confounding factors related to urbanization that may have influenced the spreading of novel coronavirus. Our epidemiological analysis uses geographical information (e.g., municipalities) and Poisson regression to assess whether both the ambient PM concentration and the excess mortality have a similar spatial distribution [129].
Regional2020The estimate of the time series slope, i.e., the expected change in the concentration associated with a time unit increase, decreased from −0.25 to −1.67 after the lockdown [130].
Country2021The model finds that there is a positive nonlinear relationship between the density of particulate matter in the air and COVID-19 transmission, which is in alignment with similar studies on other respiratory illnesses [131].
City 2021NO2 was reduced by about 49–62%, and CO and SO2 declined by about 50–58% and 70%, respectively [117].
City 2020There were significant reductions in PM2.5, PM10, CO and NO, respectively [21].
Regional2021The concentration of PM2.5 and NO2 declined by about 16% and 33%, respectively [132].
TurkeyCountry2021To determine the effects of COVID-19 measures on air quality in Turkey, for this investigation, the daily means of PM10, PM2.5, NO2, CO, O3, and SO2 air pollutant data were used [133].
Country2020By the end of April, the PM2.5 index had improved by about 35% during lockdown [134].
City2021The NO2 concentrations were reduced by about 11.8 % in the after-virus period [135].
FranceCountry2020Air quality in the Auvergne-Rhône-Alpes region, focusing on nine atmospheric pollutants (NO2, NO, PM10, PM2.5, O3, VOC, CO, SO2, and isoprene): In Lyon, the center of the region, the results indicated that NO2, NO, and CO levels were reduced by 67%, 78%, and 62%, respectively, resulting from a decrease in road traffic by 80%. However, O3, PM10, and PM2.5 were increased by 105%, 23%, and 53%, respectively [136].
RussiaCity 2020Just under half were from changes in surface transport. At their peak, emissions in individual countries decreased by –26% on average. The impact on 2020 annual emissions depends on the duration of the confinement, with a low estimate of –4% (–2 to –7%) if pre-pandemic conditions return by mid-June, and a high estimate of –7% (–3 to –13%) [137].
GermanyCity2021The concentration of NO2 reduced by about 15–25% and 34–36% from traffic sites during lockdown periods [138].
MacedoniaCountry2020PM2.5 in Kumanovo and carbon monoxide in Skopje (7% and 3% higher concentrations, respectively). The most notable decrement was for NO2, with a concentration 5–31% lower during the COVID-19 period [139].
PortugalCountry2021PM10 and NO2 concentration was reduced by about 18% and 41%, respectively [140].
Netherland Country2021NO2 and PM10 concentration was reduced by about 18–30% and 20%, respectively, during lockdown periods [141].
Poland Country2021Aerosols concentrations were reduced by about 23% and 18% in April and May, respectively [76].
SerbiaCity 2021The average daily concentrations of PM2.5, NO2, PM10, and SO2 were reduced by 35%, 34%, 23%, and 18%, respectively. In contrast, the average daily concentration of O3 increased by 8%, even if the primary precursors were reducing, thus representing a challenge for air quality management [142].
Whole Eorope Europe2021Viruses may persist in the air through complex interactions with particles and gases depending on: (1) chemical composition; (2) the electric charges of the particles; and (3) meteorological conditions, such as relative humidity, ultraviolet (UV) radiation, and temperature. In addition, by reducing UV radiation, air pollutants may promote viral persistence in the air and reduce vitamin D synthesis [143].
Europe2020The lockdown effect on atmospheric composition, in particular through massive traffic reductions, has been important for several short-lived atmospheric trace species, with a large reduction in NO2 concentrations, a lower reduction in particulate matter (PM) concentrations, and a mitigated effect on ozone concentrations due to nonlinear chemical effects [144].
Europe2020The concentration of NO2 was reduced by about 25% during lockdown periods, when compared to the same periods of previous years [145].
Table
Table 5. Literature on the impact of COVID-19 on air quality in North American countries.
Table 5. Literature on the impact of COVID-19 on air quality in North American countries.
CountryScale of the StudyPublication YearMajor Findings
USCity2020The surface air quality monitoring data from the United States Environmental Protection Agency’s (U.S. EPA) AirNow network, during the period from 20 March–5 May in 2020, to the 2015–2019 period, from the Air Quality System (AQS) network over the state of California. The results indicate changes in fine particulate matter (PM2.5) of −2.04 ± 1.57 μg m−3 and ozone of −3.07 ± 2.86 ppb. If the air quality improvements persist over a year, it could potentially lead to 3970–8900 preventable premature deaths annually (note: the estimates of preventable premature deaths have large uncertainties). Public transit demand showed dramatic declines (~80%) [147].
City2020COVID-19 prevalence and fatality (plotted as logarithm-transformed prevalence/fatality on the y-axis) as a function of mean ozone/PM2.5 AQI (plotted on the x-axis). Coefficients were not statistically significant for ozone (p = 0.212/0.814 for prevalence/fatality) and PM2.5 (p = 0.986/0.499) [148].
Country 2020The concentration of NO2 was reduced by about 25% in comparison to past years [149].
Country2020The NO2 concentration was reduced by about 5 to 49%, with a mixed impact on O3 (±20%) [24].
US2020NO2 decreased by about 9–42%, with the highest decreases (>30%) in San Jose and Los Angeles, and the lowest decreases (<12%) in Miami, Minneapolis, and Dallas [146].
US2020PM2.5 concentration was reduced by about 68% after lockdown [150].
City2020There were decreases of PM2.5 and NO2 by 36% and 51%, respectively, during lockdown [151].
City2021As per ground-based observation, it was reported that the concentration of NO2, CO, and PM2.5 dropped by about 38%, 49%, and 31%, respectively, during lockdown periods (19 March to 7 May 2020) [152].
Canada City 2020The concentration of nitrogen dioxide and nitrogen oxides reduced across Ontario [153].
Table
Table 6. Literature on the impact of COVID-19 on air quality in South American countries.
Table 6. Literature on the impact of COVID-19 on air quality in South American countries.
CountryScale of the StudyPublication YearMajor Findings
BrazilCity2020There was a substantial decrease of NO (more than 70%), CO (more than 60%), and NO2 (more than 50%). Ozone concentration increased by about 30% during partial lockdown periods, as compared to previous years [11].
City Scale2020Among CO, NO2, and PM2.5, a significant reduction was reported for CO (30–48%) [157].
City Scale2020During lockdown, CO reported the highest decline of up to 100%. NO2 decreased by about 9 to 41% [158].
EcuadorCity2020The concentration of NO2 and PM2.5 significantly decreased due to the implementation of lockdown. The concentration of PM2.5 was lower in 2020, as compared to 2018 and 2019 during the same lockdown periods i.e., March [159].
City2021There was a substantial reduction in NO during lockdown periods [160].
Regional 2020The concentration of PM2.5, PM10, and NO2 decreased by about 40%, 44% and 60%, respectively, during strict lockdown, and 69%, 58%, and 62%, respectively, during relaxed lockdown periods [155].
Country2020Air quality improved by 29–68% due to lockdown [161].
MexicoCountry2020The concentrations of NO2, SO2, and PM10 declined by about 29, 55, and 11%, respectively [154].
Table
Table 7. Literature on the impact of COVID-19 on air quality in African countries.
Table 7. Literature on the impact of COVID-19 on air quality in African countries.
CountryScale of the StudyPublication YearMajor Findings
EgyptCountry2020The whole country is improved as a result of reduced pollutant emissions, with NO2 reduced by 45.5%, CO emissions reduced by 46.23%, ozone concentration decreased by about 61.1%, and AOD reduced by 68.5%, compared to the previous two years [163].
City2021Absorbing aerosol index (AAI) and NO2 decreased by about 30% and 15%, respectively, and 33% in Cairo and Alexandria Governorate [22].
MoroccoCity Scale2020PM10 and NO2 decreased by about 75% and 96%, respectively [164].
MoroccoCountry2020COVID-19-compelled lockdown may have saved lives by restraining air pollution, thereby preventing infection. We found that NO2 dropped by −12 μg/m3 in Casablanca, and by −7 μg/m3 in Marrakech. PM2.5 dropped by −18 μg/m3 in Casablanca, and −14 μg/m3 in Marrakech. CO dropped by −0.04 mg/m3 in Casablanca, and −0.12 mg/m3 in Marrakech [165].
UgandaCity Scale2020(i) The COVID-19-induced lockdown period. The data has been compared with the same period of the previous year. Promising and notable observations were made in terms of the AQI of Kampala [166].
NigeriaCity Scale2021The lockdown resulted in a decrease of SO2 and NO2 across the cities. For example, 1.1% and 215.5% of NO2 and SO2, respectively, from the city Port Harcourt [167].
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Addas, A.; Maghrabi, A. The Impact of COVID-19 Lockdowns on Air Quality—A Global Review. Sustainability 2021, 13, 10212. https://doi.org/10.3390/su131810212

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Addas A, Maghrabi A. The Impact of COVID-19 Lockdowns on Air Quality—A Global Review. Sustainability. 2021; 13(18):10212. https://doi.org/10.3390/su131810212

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Addas, Abdullah, and Ahmad Maghrabi. 2021. "The Impact of COVID-19 Lockdowns on Air Quality—A Global Review" Sustainability 13, no. 18: 10212. https://doi.org/10.3390/su131810212

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